In local area network priority systems, and in particular in token passing systems, such as a token ring, priority of access is determined by the position of the token. Generally, the token circulates through each node on the ring and, if a node desires to transmit obtains a free token, it can then transmit its message over the shared transmission medium. Such systems are known as matched priority systems. In a desirable local area network priority system, whilst normally permitting matched priority in one mode of operation, it should also permit simultaneous operation of a multiple priority mode which allows rapid access of higher priority messages which are independent of maximum data packet sizes. Such a multiple priority system should be implemented using hardware and software and should have applicability in local area networks where data is serially passed i.e. in token-passing buses and in particular token-passing rings and also parallel processing systems where information is transmitted in parallel. A multiple priority system could also permit dynamic reconfiguration of node priorities as and when required. This would permit flexibility of operation in allowing change in the node priority as and when required to allow priorities to be allocated to individual messages and such a scheme can involve the use of a network manager.
In existing local area network priority systems, and in particular in token passing systems such as token rings, or token buses having multiple nodes coupled to the medium, the access time (for a given medium data rate) of each node to transmit messages onto the medium is dependent on the maximum packet size used and the priority of the node requiring access. In such system a node must wait for a free token which has a priority equal to or less then the priority of the node desiring to transmit before it can transmit information in the form of a data packet. Such priority systems are disclosed and proposed in IBM token ring information and in the IEEE 802.5 report on token ring standards. However, if a real-time local area network is to permit a wide range of packet sizes to be used for example large data packet for file transfers, and short packet control purposes, and short packets for control information then some nodes may face unacceptable time delays. Thus time critical messages may not have access to the network when desired and other nodes may be limited to short packet lengths. Further conventional token passing networks tend to allocate network capacity a frame at a time. This means that several short messages in one node could be transmitted before a long message in another node which had started earlier could be transmitted.
An object of the present invention is to obviate and mitigate the aforesaid disadvantages of the aforementioned local area network priority system.